Hoerner, Sebastian von (1919–2003)

German radio astronomer who worked at Green
Bank with Frank Drake and later at the
Astronomisches Rechen-Institut (Astronomical Calculation Institute) in Heidelberg.
He was active in the 1960s in discussions about SETI,
the number of advanced civilizations in the Galaxy, and the feasibility
of interstellar travel. In a 1961 paper,1
he discussed various factors that might affect the longevity of technological
races and what this in turn would mean for the chances of success of SETI
programs. He was not generally optimistic about the long-term survival prospects
of machine-using species, noting that science and technology on Earth were
often driven by:

... the fight for supremacy and by the desire
for an easy life... [These forces] tend to destroy if they are not controlled
in time: the first one leads to total destruction and the second one leads
to biological or mental degeneration.

The latter view he shared with Ronald Bracewell.
He also considered the possibility that a species might lose interest in
material progress or be completely wiped out. In von Hoerner's opinion,
it was feasible that a succession of technological species might rise and
fall in the lifetime of a planet, each new emergence taking hundreds of
millions of years. His calculations showed that if the average longevity
of an advanced race is 6,500 years and each habitable planet produces an
average of four technological species, then perhaps one in every 3 million
stars might support a race capable of interstellar communication. The mean
distance between such worlds would be about 1,000 light-years. von Hoerner
was among those, including John Pierce and
Edward Purcell, who regarded interstellar
travel as an unlikely proposition. Even assuming spacecraft could be accelerated
to almost the speed of light, at which relativistic time
dilation would come into play, von Hoerner argued that it would never
be acceptible for travelers to cross a thousand ly or more to reach another
inhabited planet, only to return home and find that many centuries had passed
by.2